ORIGIN OF THE LAND VERTEBRATES 97 



characteristic organ, though it has disappeared in an 

 erratic and unaccountable way in many genera. The 

 air-bladder of the Teleostean fish, while exhibiting 

 considerable variations in its detailed structure and 

 anatomical relations, is essentially a hollow thin- 

 walled sac produced as a large pouch from the 

 oesophagus, and often communicating with the throat 

 by means of a duct, the pneumatic duct. The bladder 

 itself is in all cases save one disposed as a spacious 

 air-containing bag dorsally to the oesophagus and 

 viscera, being covered over by the dorsal peritoneum. 

 In Polypterm, the Bichir of the Nile and West 

 African rivers, a peculiar and primitive fish, both 

 bladder and duct are situated ventrally to the oeso- 

 phagus, but in all other fishes the bladder has 

 travelled on to the dorsal surface, though the duct 

 frequently remains in its original ventral situation, 

 travelling dorsally round the oesophagus to the 

 bladder (see Fig. 20 B). 



In modern Teleosts the air-bladder is aerostatic 

 in function, the lining secreting a mixture of oxygen 

 and nitrogen gas, the nitrogen being usually greatly 

 in excess. The function of the bladder in these forms 

 consists in the power it gives the organism of counter- 

 acting the effects of varying pressure at different 

 depths of water. The arterial blood supply to 

 the air-bladder in the ordinary Teleost or Bony-fish 

 (Fig. 20 D) is given off from the coeliac artery, and the 



s. A. K. 7 



